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A prehistoric giant millipede at the Field Museum of Natural History in Chicago.

The first insects – or, at least the precursor to modern insects – go all the way back to 400 million years ago, says Texas A&M Agrilife Extension Office insect specialist Wizzie Brown. She says an extinct group of prehistoric, dragonfly-like bugs called Meganeura buzzed about similar to the dragonflies of today, except for one major difference: they were huge!

“We’re talking two-foot wingspan,” Brown says.

Giant scorpions also roamed about – on land and in water. The so-called “sea scorpion” was eight feet long and carnivorous: it ate fish, and even other sea scorpions.

Brown says there are various theories about why these groups of insects don’t exist anymore. One theory is that oxygen level in the atmosphere is different today than it was millions of years ago, and those species struggled to adapt to the change. Another theory is that evolution weeded out insects that were less nimble and had trouble maneuvering their large bodies.

“It favored those insects that were smaller,” she says. “That’s the cool thing about science, we get to discover new things as time goes on.”